In the field of actuators for driving home-automation screens, it is known to use a braking device, for, on one hand, braking the movement on approaching an obstacle such as an end-of-travel stop, and, on the other, locking the load when the motor is not operating. In the case of a home-automation screen such as a small blind or curtain, the lightness of the home-automation screen means that the torque applied by the screen on the motor, particularly in the descent phase, is low. In this way, it is not necessary to equip the actuator with an external braking device, since the motor may be braked itself by short-circuiting or reluctance. This braking may be completed by the use of an irreversible reduction gear which does not transmit the torque applied by the load to the motor.
For larger screens, it is however essential to use an actuator comprising a mechanical or electromechanical brake external to the motor, to ensure personal protection.
Such an actuator, described in EP-A-0 508 949, comprises an asynchronous type electric motor, a reduction gear and a plate brake suitable for moving under the action of the magnetic field produced by the motor windings. Asynchronous motors have the advantage of being suitable for being connected directly to the mains but the variable-speed control thereof is complex since it involves adjusting the frequency of the control signal while retaining the power required for operation. This generally involves oversizing the motor.
To remedy this problem in respect of the variable-speed control of the asynchronous motor, it is known to equip the actuators for driving home-automation screens with direct-current, brush and collector or electronic switching motors. In the latter, the rotor is formed by one or a plurality of permanent magnets, whereas the stator is formed by a set of coils; switching the power supply of the various coils makes it possible to create a rotating magnetic field which rotates the magnets of the rotor. Electronic switching motors do not use a brush-collector system, rendering such motors more robust. However, this type of motor requires a reliable and complex electronic control unit.
In most actuators for driving a home-automation screen, it is necessary to know the position of the output shaft at all times. This is carried out either by a mechanical counting unit, driven by a rotary element rigidly connected to the motor output shaft or by an electronic counting unit, particularly counting the number of motor switches. In the case of counting at the motor output or at the rotor, this accurately reflects the position of the motor, but is imprecise in relation to the actual position of the load. Conversely, counting closer to the load, for example at the rotation of a home-automation screen winding tube, is more reliable in respect of the actual position thereof, but requires a more complex design.
The integration of an electronic switching motor in an actuator for driving a home-automation screen logically prompts the use of electronic referencing of the rotor position. However, if the motor is used with a brake operating in on/off mode, such as a plate brake, the motor is liable to operate as a generator on some operating ranges, while it is driven by the load, giving rise to heat dissipation issues.